The instant invention relates to an apparatus useful in collecting hydrocarbons emanating from an earth formation, and particularly to an apparatus for collecting small concentrations of hydrocarbons dissolved in a body of water overlaying an earth formation.
There are many systems useful for collecting samples and analyzing the samples for hydrocarbons. Such systems have been found to be useful for predicting the hydrocarbon potential of earth formations and may be utilized in exploring for oil beneath the ocean floor. In general such systems continuously sample the ocean water for methane, from which the hydrocarbon or petroleum potential of the ocean floor in the vicinity of sample collection may be predicted.
When methane has been found, it may be analyzed to determine the stable carbon isotope ratio. It is known that samples originating from the biogenic decomposition of organic matter will have a delta .sup.13 C range of -65 per thouand to -100 per thousand while methane originating from the thermogenic decomposition of organic matter will have a delta .sup.13 C range of -20 per thousand to -50 per thousand. (The delta .sup.13 C measurement is defined in Petroleum Formation and Occurrence, B. P. Tissot, D. H. Welte, Springer-Verlag, N.Y. (1978) at p. 88 as: ##EQU1## Thus, the stable carbon isotope ratio allows the explorer to predict whether or not the methane is derived from a petroleum source or as a result of the more recent biogenic decomposition of animal or vegetable matter. However, a relatively small amount of thermogenic methane mixed with biogenic methane can go unnoticed.
In British Pat. No. 2,074,726 a method and apparatus for predicting hydrocarbon potential of an earth formation under a body of water is disclosed. The apparatus disclosed in the British patent includes a network of adsorbent traps, furnaces and cryogenic traps which are utilized to separate and collect the components of a flowing sample that has been obtained by a sampling device positioned in the vicinity of the earth formation. The method and apparatus, while generally useful for examining the stable carbon isotope distribution of any hydrocarbons collected, is specifically concerned with information derived from the analysis of methane. It is not suggested in this patent that important information can be obtained by separating the methane from any higher hydrocarbons contained in the sample and separately analyzing both the methane and the higher hydrocarbons. This is an important failing since methane is more likely to be derived from the decomposition of animal and vegetable wastes, while the higher hydrocarbons are more generally found to be of petroleum origin.
Furthermore, the network disclosed in the British patent is indicated as terminating at a vent, through which the non-collected components of the sample are removed. It has been found that carbon dioxide from the atmosphere is able to enter the network at the vent and contaminate the sample. This a problem in view of the method of collecting the methane sample to analyze the stable carbon isotope ratio thereof. That is, methane is collected as a sample by oxidizing methane to carbon dioxide (i.e., methane-derived carbon dioxide) and condensing the carbon dioxide in a cryogenic trap. Since atmospheric carbon dioxide may have a delta .sup.13 C value of about -7 per thousand, the contamination of the condensed carbon dioxide would indicate that the methane is of thermogenic origin when in actuality it is of biogenic origin.
It is also apparent that the condensation of the oxygen of the sample in the cryogenic traps was of concern in the design of the apparatus of the British patent. As disclosed in the British patent, the methane component is condensed at a temperature within the range of from about -160.degree. to about -180.degree. C. To achieve this temperature either liquid argon or a mixture of isopentane and liquid nitrogen is utilized as the cryogenic medium. At lower temperatures such as -196.degree. C. (the temperature of liquid nitrogen) oxygen may also condense. The condensed oxygen is prone to vaporize when the cryogenic trap is removed from the network for analysis of the sample contained therein. If sufficient condensed oxygen is present, the trap may burst and expel the methane sample. There are disadvantages in the utilization of either liquid argon or the mixture of isopentane and liquid nitrogen. Liquid argon is less available than liquid nitrogen and is approximately four times as expensive. The mixture of isopentane and liquid nitrogen is difficult to make up and use as a cryogenic medium. If not for the problem of condensing oxygen in the cryogenic traps at a temperature of -196.degree. C., it would be preferable to use liquid nitrogen as the cryogenic medium.
Finally, the apparatus disclosed in the British Patent does not provide for the restriction of flow prior to the above network. Thus, the apparatus will not operate at a variable pressure. It would be desirable to utilize an apparatus for collection of hydrocarbons that could be adapted to operate at the most efficient pressure for separating the various components of the sample into fractions that provide useful information about the origin of the hydrocarbons contained therein.
Thus it is one object of the invention to provide a method for separating and collecting methane and higher hydrocarbons from a sample for separate analysis to determine whether either the methane or the higher hydrocarbons, or both are of thermogenic origin.
It is another object of the invention to provide a method for collecting and examining gaseous samples including methane wherein methane is separated from the gaseous sample by oxidation to carbon dioxide and cryogenic condensation of the carbon dioxide and wherein the condensed carbon dioxide is protected from contamination by atmospheric carbon dioxide.
It is another object of the invention to provide a method useful for the collection of hydrocarbons which method may be carried out at any pressure or flow which will maximize the efficiency of separating and collecting the various hydrocarbon fractions found in said sample.
It is another object of the invention to provide a method useful for the collection of hydrocarbons which uses a sample collection device that may be operated at the various pressures and flows emanating from such sample collection device.
Other objects and advantages of the instant invention will become apparent from the following description taken in connection with the accompanying figure, in which the preferred embodiment of this invention is set forth by way of illustration and example.